Three-dimensional topography distribution of cracks is one of the important measurements of physical quantities in a hydraulic fracturing test of rocks. Presently, an observation of the surface cracks in hydraulic fracturing test of rocks mainly depends on the means of scanning electron microscopes, etc., while an observation of the internal three-dimensional topography mainly depends on the x-ray CT imaging technology. The x-ray CT imaging technology has a higher positioning precision, but a limited imaging precision, for cracks in hydraulic fracturing test of rocks. Presently, industrial CT is incapable of imaging cracks with widths less than 0.1 mm in a rock sample which has a diameter of 100 mm, whereas about 86% of cracks have a width less than 0.1 mm in a hydraulic fracturing test, hence, industrial CT is incapable of effectively observing a majority of cracks in a hydraulic fracturing test of a rock with a diameter of 100 mm, losing information of a number of existent cracks.
Therefore, current methods for observing cracks in a hydraulic fracturing test of rocks fail to satisfy the needs of observing three-dimensional topography of cracks in a hydraulic fracturing test of rocks.